IP based Push-to-Talk (PTT) systems are often referred to as PTT over Cellular (PTToC) systems because a dominant use of these systems is with mobile phones that use a mobile phone app so that a plurality of mobile phones can communicate with each other. In reality, the cellular phone connection serves to provide an IP connection from the mobile phone device to a PTT server, which is often resident in a cloud computing environment, such as Amazon Web Services (AWS). Any terminal device, such as a computer and the like, that can run a PTT application and connect via IP to the PTT Server could be used in an IP-based PTT application.
A central tenet of PTT applications is that conversations are half-duplex, which is enforced by the PTT Server employing a floor control function, which determines which user is granted transmit access (i.e. given the floor) to a PTT channel. This makes PTT applications ideal for group communications, ensuring more orderly conversations and eliminating the need for users to address their transmissions to specific users. This half-duplex characteristic arose from the use of two-way radio (LMR) usage, where it was an inherent limitation of the two-way radio technology. Digital radio systems introduced floor control functions, where users pressing the transmit button were given a denial indication if someone else had been granted access to the floor. This indication method is used today in PTToC systems.
PTT systems typically make use of the talk-group concept. A talk group is an administrative entity that is defined by a name and a set of assigned users. When a user transmits on a talk-group, all other assigned users of that talk-group will receive his or her transmission.
A natural extension to PTToC systems has been the ability to integrate two-way radio systems, allowing PTT operation across both radio and IP terminals. This is typically accomplished by employing a gateway function at the PTT server that translates a radio-oriented protocol to one usable by the PTT server. Radio integration also requires an interface at the radio system itself. There are standard digital interfaces available, such as APCO P25 based IS SI or ETSI DMR based AIS, for interfacing to digital radio systems. However, most integrations today use a simple Radio over IP (RoIP) protocol, which requires a simple interface at the radio, usually just to receive audio, to transmit audio, and to provide a push-to-talk signal.
There are no neutral standards for RoIP protocols. Many vendors simply digitize the audio signal with a standard codec (e.g. G.711, OPUS, etc.) and send it, without any signaling or link status messaging. The digitized voice is usually wrapped in RTP packets and transported via UDP/IP. Some vendors, such as JPS, have added a link status keep alive message in their proprietary protocol.
Software Defined Radio (SDR) is a radio system architecture where many radio functions traditionally implemented with specialized and dedicated hardware are instead implemented on a general-purpose computer (e.g. a PC). The general-purpose computer is connected to a small, specialized hardware receiver device, which is typically much smaller and cheaper than an entire traditional radio.
This SDR system provides numerous benefits. The system cost can be reduced because of Moore's Law based costs associated with general purpose computing. The radio application is more configurable, programmable, and manageable, compared to standard radio. The bulk of the radio functionality can be implemented remotely from the receiver hardware itself. Since donor radios have typically played a large role in PTToC architectures, SDR may impact many factors in PTToC applications. Currently, commercially available SDR devices are usually hobbyist or ham radio grade equipment and laboratory-based equipment
There are many SDR software applications available, most of them free and/or open source. SDR receive-only applications are currently more viable than transmit applications, because the hobbyist-oriented market is not sufficiently large to justify manufacturers' expense for certifications required to bring commercial radio transmission equipment to market. An exception to this is SDR products that are exclusively targeted at amateur radio operators.
An important function of radio communications is two-tone audio paging, also known as fire-tones. In the US, many fire agencies within a region share a common radio channel. When a new dispatch call goes out on the radio, a sequence of audio tones is sent on the radio channel, prior to the dispatcher speaking, to specify which fire stations in the region are being asked to respond. Firefighters often program a squelch into their radios that is broken only when the tone-out for their station is received on the radio.